In the rapidly expanding fields of photonics and optoelectronics, optical channel waveguides are becoming increasingly important for on-chip, chip-to-chip, and board-to-board interconnections. Optical waveguides are systems or materials designed to direct and/or process light and may be characterized as either passive or active.
A passive optical waveguide is a physical structure which is capable of transmitting light while at the same time confining it to a particular region in space. In a passive waveguide, this confinement may be achieved by altering the refractive index within the region of the waveguide, the core, so that it is higher than that of the surrounding material, the cladding. Light will tend to propagate within the region of higher refractive index and so can be moved from a starting point to an ending point along a path which is defined by the waveguide region.
In an active optical waveguide, the light is not simply passively transported from one point to another, but is also processed while traveling within the guide. More particularly, these waveguides have nonlinear optical properties which are capable of changing the nature of the light (i.e. phase, amplitude, or frequency) as the light passes through the waveguide. By altering the properties of the light within a waveguide or a waveguide region, it is possible to encode and decode information and to route it as desired.
Organic polymeric materials offer important advantages for making both active and passive waveguides because they are thermally stable at high temperatures and may be readily processed and chemically modified to obtain the appropriate linear and nonlinear optical properties necessary to transmit and control light. In addition, photochromic organic polymers are of particular interest because of their ability to change refractive index upon irradiation with light at a wavelength within their respective absorption bands.
Conventional polymeric photochromic compositions include stilbene- and diazobenzene-type dye compounds, which have strong electron donors and acceptors, as described in U.S. Pat. No. 5,142,605 to Diemeer et al. and U.S. Pat. No. 5,541,039 to Mcfarland et al. These dyes are characterized by their intense absorptions in the visible region and second-order polarizability, making the materials containing them suitable for active waveguides. When the materials are irradiated at a wavelength within their corresponding absorption bands, the dyes undergo conformational isomerization. This causes the absorption band to shift to shorter wavelengths and is accompanied by a decrease in intensity. Consequently, a decrease in refractive index of the polymeric material is observed. These photochromic compositions are characterized as "negative systems" due to the decrease in refractive index that occurs upon exposure to light.
The aforementioned change in refractive index can serve as a mechanism for patterning optical waveguides. By exposing some region of the photochromic polymeric film to sufficient intensity within the absorption band, the refractive index of the exposed region is reduced. By not exposing a narrow stripe, 1 to 2 .mu.m for example, a higher index can be maintained in the unexposed area, creating a waveguide, wherein the unexposed area serves as the waveguide core. Unfortunately, though, over time, as light propagates through the core, the same irradiation process (often referred to as "photobleachinig") that created the waveguide structure in the first place can eventually cause the refractive index of the core to decrease, and the waveguide is "bleached" away.
Another disadvantage with waveguides formed from conventional negative photochromic compositions is that the waveguides tend to be unstable over time. In particular, the photochemical isomerization mechanism discussed above is reversible, and the exposed material will slowly revert to its original state. Thus, the refractive index of the exposed region will return to its original higher value, and the usefulness of the material as an optical waveguide will terminate.
For example, Diemeer et al. reported in Electron. Lett. 26, 379 (1990) the use of a polymer containing a dimethylamino nitrostilbene (DANS) electro-optical side chain to fabricate a waveguide. By selective photobleachinig of the DANS chromophore at 431 nm, the transform of the DANS was converted to the cis form, thereby reducing the refractive index of the exposed polymer. However, this reaction is thermally reversible, and cis-DANS will slowly transform back to the trans form resulting in the loss of optical waveguiding properties.
Likewise, Beeson et al. reported in Appl. Phys. Lett. 58, 1955 (1991) a system based on the photobleaching of a nitrone compound, 4-N,N-dimethylaminophenyl)-N-phenyl nitrone (DMAPN), at 361 nm. Upon irradiation, the nitrone rearranges to an oxaziridine, and the absorption maximum shifts to 274 nm, resulting in a decrease in refractive index. However, as discussed by Smets et. al. in J. Polym. Sci. Polym. Chem. Ed. 14, 2983 (1976), the nitrone system is very unstable, and the oxaziridine readily reverts back to the nitrone in the absence of light.
More recently, a system based on the photobleaching of o-nitrostilbene at 488 nm was reported by I. A. McCulloch in Macromnolecule 27, 1697 (1994). Upon irradiation, the refractive index of the material decreases due to transformation of the dye to isatogen. This system was found to be 100 times more photosensitive than the DANS system. However, based on a study of the photoreaction by Spitter et al., at J. Org. Chem. 20, 1086 (1955), the exposed region was found to slowly regain its original refractive index resulting in waveguide instability.
As is apparent from the prior art, the photobleaching process is not entirely controllable, and waveguides prepared using this method deteriorate over time. This is a result of either relaxation of the conformational change back to the original state, or continued bleaching of the core region through normal use. Thus, designers have recently turned to alternative methods to create waveguides which have included embossing and reactive ion etching (RIE). However, such processes are complex and difficult to perform.
Based on the aforementioned problems associated with known techniques for creating optical waveguides, there is an apparent need for the development of more stable photochromic waveguide structures that can be formed using simple photolithographic methods. In particular, a need exists for the identification of "positive" photochromic compositions, wherein the refractive index of the light-exposed region increases. Waveguide systems formed from such positive photochromic compositions would avoid the "bleaching" problem making them more stable over time. In addition, a need exists for positive photochromic compositions that undergo an irreversible photochemical change, thereby eliminating the problems associated with light-initiated isomerization. Replacing traditional procedures for forming polymeric waveguides which require multi-step processes involving photolithography followed by plasma etching, wet chemical etching, or thermal heating with a simpler process is also highly desirable.
Moreover, there is a need for photochromic compositions for use in waveguide systems wherein the materials produced during irradiation have improved second-order nonlinear optical (NLO) properties. Larger NLO effects translate into lower power requirements and higher performance in active waveguides. Such organic photochromic compositions should exhibit negligible second-order polarizability before irradiation and large second-order polarizability after exposure.